Method and apparatus for ultrasound velocity measurements in drilling fluids

ABSTRACT

The disclosure relates to methods and apparatus for determining the velocity of an ultrasound pulse in drilling fluids in downhole environments. A method for determining a velocity of ultrasound propagation in a drilling fluid in a downhole environment includes emitting an ultrasound pulse into the drilling fluid in a borehole using a first ultrasound transducer ( 37 ); detecting the ultrasound pulse after the ultrasound pulse has traveled a distance (d); determining a travel time (t) required for the ultrasound pulse to travel the distance (d); and determining the velocity of ultrasound propagation from the known distance (d) and the travel time (t). An apparatus for determining a velocity of ultrasound propagation in a drilling fluid in a downhole environment includes a first ultrasound transducer ( 37 ) disposed on a tool; and a circuitry ( 82 ) for controlling a timing of an ultrasound pulse transmitted by the first ultrasound transducer ( 37 ) and for measuring a time lapse between ultrasound transmission and detection after the ultrasound pulse has traveled a distance (d).

BACKGROUND OF INVENTION

Accurate borehole dimension data are important for well logging and wellcompletion. Measurements performed by many logging tools, whetherwireline, logging-while-drilling (LWD), or measurement-while-drilling(MWD) tools, are sensitive to borehole sizes or tool standoffs.Therefore, accurate borehole dimension information may be required tocorrect measurements obtained with these tools. Furthermore, informationregarding a borehole dimension is used to determine well completionrequirements, such as the amount of cement required to fill the annulusof the well. In addition, borehole dimension data may be used to monitorpossible borehole washout or impending borehole instability such that adriller may take remedial actions to prevent damage or loss of theborehole or drilling equipment.

Borehole dimensions, such as diameter, may be determined with variousmethods known in the art, including ultrasound pulse echo techniquesdisclosed by U.S. Pat. Nos. 4,661,933 and 4,665,511. Such ultrasoundmeasurements rely on knowledge of the velocity of the ultrasound pulsein the particular medium, e.g., drilling fluids.

However, the velocity of an ultrasound pulse, typically, is not easilymeasured in a wellbore. Instead, the velocity of an ultrasound pulse inthe well is typically extrapolated from an ultrasound velocitymeasurement made at the surface based on certain assumptions concerningthe mud properties under downhole conditions. Such assumptions may notbe accurate. Furthermore, mud properties in a drilling operation maychange due to changes in the mud weight used by the driller, pumppressure, and mud flow rate. In addition, the drilling mud may becomecontaminated with formation fluids and/or earth cuttings. All thesefactors may render inaccurate the velocity of an ultrasound pulseestimated from a surface determination.

Therefore, there is a need for improved methods and apparatus for themeasurement of ultrasound velocity in downhole environments.

SUMMARY OF INVENTION

In one aspect, the invention relates to methods for determining avelocity of ultrasound propagation in a drilling fluid in a downholeenvironment. A method according to one embodiment of the inventionincludes emitting an ultrasound pulse into the drilling fluid in aborehole using a first ultrasound transducer (37); detecting theultrasound pulse after the ultrasound pulse has traveled a distance (d);determining a travel time (t) required for the ultrasound pulse totravel the distance (d); and determining the velocity of ultrasoundpropagation from the distance (d) and the travel time (t).

In another aspect, the invention relates to apparatus for determining avelocity of ultrasound propagation in a drilling fluid in a downholeenvironment. An apparatus according to the invention includes a firstultrasound transducer (37) disposed on a tool; and a circuitry (82) forcontrolling a timing of an ultrasound pulse transmitted by the firstultrasound transducer (37) and for measuring a time lapse betweenultrasound transmission and detection after the ultrasound pulse hastraveled a distance (d). The apparatus may further comprise a secondultrasound transducer (39). The first and second ultrasound transducer(37 and 39) may be arranged across a fluid channel. Alternatively, theymay be arranged on a surface of the tool. Furthermore, the first and thesecond ultrasound transducer (37 and 39) may be adjacent each other witha front face (37 f) of the first ultrasound transducer (37) and a frontface (39 f) of the second ultrasound transducer (39) offset at apredetermined offset distance (ΔD_(f)).

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a logging tool disposed in a borehole.

FIGS. 2A and 2B is illustrate a prior art method for determining avelocity of an ultrasound pulse.

FIG. 3 shows an apparatus for measuring the velocity of an ultrasoundpulse according to one embodiment of the invention.

FIG. 4 shows a recording of ultrasound measurement using the apparatusshown in FIG. 3.

FIG. 5 shows an apparatus for measuring the velocity of an ultrasoundpulse according to another embodiment of the invention.

FIG. 6 shows a recording of ultrasound measurement using the apparatusshown in FIG. 5.

FIG. 7 shows borehole having an apparatus for measuring the velocity ofan ultrasound pulse according to another embodiment of the invention.

FIG. 8 shows the side view of borehole having an apparatus for measuringthe velocity of an ultrasound pulse according to another embodiment ofthe invention shown in FIG. 7.

FIG. 9 shows a cross section of a tool having an apparatus for measuringthe velocity of an ultrasound pulse according to the embodiment of theinvention shown in FIG. 3.

FIG. 10 shows a schematic of a control circuitry according to oneembodiment of the invention.

DETAILED DESCRIPTION

The invention relates to methods and apparatus for determiningultrasound velocity in drilling muds under downhole conditions. Methodsfor determining the velocity of an ultrasound pulse, in accordance withone embodiment of the invention, measure the time (“travel time”) ittakes the ultrasound pulse to travel a known distance (d) in the mudunder downhole conditions. Once the velocity of an ultrasound pulse isknown, it may be used to calculate downhole parameters, e.g., boreholediameters. Alternatively, the downhole parameters may be determined,according to another embodiment of the invention, by using twoultrasound transducers disposed at different distances from the targetsurface.

Methods and apparatus of the present invention are useful in welllogging. Embodiments of the invention may be used in a wireline tool, anMWD tool, or an LWD tool. FIG. 1 shows a logging tool (1) inserted in aborehole (3). The logging tool (1) may include various devices, such asan ultrasound transducer (5), for measuring the borehole or formationproperties. For example, the ultrasound transducer (5) may be used todetermine the borehole radius by measuring the distance between theultrasound transducer (5) and the borehole's interior surface. Thedistance may be determined from the travel time of the ultrasound pulseand the velocity of the ultrasound pulse in the mud.

The travel time of an ultrasound pulse is typically measured by firingthe ultrasound pulse at a reflective surface and recording the time ittakes the ultrasound pulse to travel to the reflective surface and backto the transducer. FIG. 2A illustrates a schematic of ultrasound waves(shown in continuous lines) traveling to a reflective surface (21) andback (shown in dotted lines), using a conventional setup. The ultrasoundwave may be generated by an ultrasound transducer (22), which typicallycomprises a piezoelectric ceramic or a magnetostrictive material thatcan convert electric energy into vibration, and vice versa. Theultrasound transducer (22) may function both as a transmitter and areceiver. The transducer preferably is configured such that it emits apulse in a collimated fashion in a direction substantially toward thereflective surface with little or no dispersion. The transducersdiscussed herein may, for example, be transducers such as thosedescribed in U.S. Pat. No. 6,466,513 (Acoustic sensor assembly, Pabon etal.)

FIG. 2B shows a typical recording of ultrasound vibration magnitudes asa function of time as detected by the transducer (22). Two peaks arediscernable in this recording. The first peak (23) arises from the frontface echo, which is the vibration of the ceramic element when theultrasound pulse leaves the front face of the transducer (22). Thesecond peak (24) results from the echo returning to the transducer (22).Thus, the time period between the detection of the first and the secondpeaks represents the travel time for the ultrasound pulse from thetransducer (22) to the reflective surface (21) and back. This time isequal to twice the time it takes the ultrasound pulse to travel from thetransducer (22) to the reflective surface (21). The time lapse may bemeasured using any analog or digital timing device adapted to interfacewith, for example, the circuitry that controls the ultrasoundtransducers.

Once the travel time is determined, it is possible to determine thedistance between the transducer (22) and the reflective surface (21) ifthe velocity of the ultrasound pulse in the medium is known. As notedabove, the velocity of an ultrasound pulse in a drilling fluid in theborehole is typically measured at the earth surface. The velocity thusdetermined is then corrected for effects of temperature, pressure, andother factors expected in downhole environments. However, this approachdoes not always produce an accurate velocity of the ultrasound pulse indownhole environments due to errors in predicting the downholeconditions (e.g., temperature and pressure) or due to other unexpectedfactors (e.g., the drilling fluid may mix with formation fluids and/orearth cuttings). In order to obtain reliable velocity of an ultrasoundpulse, it is desirable to measure the velocity of the ultrasound pulsesin situ.

One or more embodiments of the invention relate to methods and apparatusfor determining the velocity of an ultrasound pulse in downholeenvironments. FIG. 3 shows an apparatus according to one embodiment ofthe invention. The apparatus is shown disposed in a borehole drilledthrough a formation 38, and includes a tool collar and chassis (27)defining a mud channel (29) therein. The area between the apparatus andthe formation is known as the annulus 36. The mud channel (29) istypically approximately 5 cm in diameter and provides a path throughwhich drilling mud may be pumped into the borehole. The mud then returnsto the surface, together with drilling cuttings and other contaminants,via the annulus 36.

The apparatus of this embodiment includes a first ultrasound transducer(37) and a second ultrasound transducer (39) located across the mudchannel (29) and facing each other. The transducers are separated fromthe mud channel by a thin interface 40, which may be metal andapproximately 5 mm thick. The thin interface protects the transducersfrom the contents of the mud channel while permitting transmission andreception of ultrasound pulses there through. Apparatus 27 furtherincludes circuitry for controlling the ultrasound transducers and forrecording the received signal as shown and described in connection withFIG. 10. The first ultrasound transducer (37) is used as a transmitter,while the second ultrasound transducer (39) is used as a receiver. Thisparticular configuration is referred to as a “pitch-catch”configuration. This embodiment may be incorporated into any logging toolto determine the velocity of an ultrasound pulse in the mud in downholeenvironments.

A method for measuring the velocity of an ultrasound pulse using theapparatus (27) includes the following steps. First, an ultrasound pulseis transmitted from the first ultrasound transducer (37) into the mudchannel (29). Then, the time that takes the ultrasound pulse to travelfrom the first ultrasound transducer (37) through the mud in the channelto the second ultrasound transducer (39) is measured. Finally, thetravel time is used to determine the velocity of the ultrasound pulsebased on the diameter of the mud channel (D_(mc)).

FIG. 4 shows a typical recording from a measurement using an apparatusin the pitch-catch configuration shown in FIG. 3. Trace (41) is arecording from the first ultrasound transducer (37). This trace includesa peak (43), which indicates the time when the ultrasound pulse leavesthe front face of the first ultrasound transducer (37). Trace (42) is arecording from the second ultrasound transducer (39), which includes apeak (44) that resulted from the detection of the ultrasound pulse bythe second ultrasound transducer (39). The time lapse (t) between peak(43) and peak (44) represents the time required for the ultrasound pulseto travel from the first ultrasound transducer (37) to the secondultrasound transducer (39). Because the distance between the twotransducers is known, the velocity of the ultrasound pulse in the mudchannel can be computed from the time lapse between the detection of thefirst peak (43) and the second peak (44).

FIG. 5 shows another embodiment of the invention having a singleultrasound transducer (37) that functions to both transmit and receiveultrasound pulses. This particular configuration is referred to as a“pulse-echo” configuration. In this embodiment, an ultrasound pulse isfirst transmitted substantially perpendicular to the mud channel (29).The ultrasound pulse bounces off the mud-metal interface at theinterface (40), and the reflected ultrasound pulse (echo) is detected bythe ultrasound transducer (37).

FIG. 6 shows a typical recording using the pulse-echo apparatus shown inFIG. 5. In FIG. 6, the first peak (61) reflects the time when theultrasound pulse leaves the front face of the ultrasound transducer (37)and the second peak (62) indicates the time when the ultrasound pulse(echo) reaches the transducer (37) after having been reflected by themetal interface (40) on the opposite side of the mud channel. The timelapse (t) between the first and the second peaks is the time it takesthe ultrasound pulse to travel twice the diameter of the mud channel(D_(mc)). The velocity of propagation of the ultrasound pulse within themud channel (29) is computed by dividing the mud channel diameter(D_(mc)) by one half the travel time (t/2).

The “pitch-catch” embodiment of FIG. 3 and the “pulse-echo” embodimentof FIG. 5 have various relative advantages and disadvantages, and thusan appropriate configuration may be chosen for a desired application. Inthe case of the pulse-echo configuration, the sound wave emitted by thetransmitter (37) has to go through three interfaces before beingdetected by the same sensor. The first interface is metal-mud, thesecond interface is mud-metal in the opposite wall of the mud channel,and the last interface is the mud-metal interface back at the transducer(37). Sound wave travel is governed by the laws of transmission andreflection. Given the difference in acoustic impedance between the mudand metal, most of the energy is going to be reflected back at thetransducer on the first interface. The little energy transmitted(transmission coefficient, T˜0.09) has then to travel across the mudchannel, being attenuated by the mud and be reflected into the secondinterface. Here more of the signal is recovered (reflection coefficient,R˜0.8). Then, the reflected signal must travel back to the originalinterface, suffering the same attenuation as in the first leg across.Finally, the wave must cross the mud/steel interface and reach thetransducer, although this time the transmission coefficient is favorableand thus there is almost no loss.

The pitch-catch configuration has the advantages that the attenuation ofthe mud channel medium is encountered only once, and that there are twointerfaces for the pulse to cross rather than three. Thus, it is easierto detect the pulse of interest. The pulse-echo configuration, however,has the advantage of more simple construction.

The apparatus shown in FIGS. 3 and 5 are useful for determining thevelocity of an ultrasound pulse in the mud before the mud iscontaminated with earth cuttings or formation fluids. In bothconfigurations, the known diameter of the mud channel (D_(mc)) is usedto calculate the velocity of the ultrasound pulse. One skilled in theart would appreciate that these configurations can be easily adapted tomeasure the velocity of an ultrasound pulse in the annulus, instead ofin the mud channel. For example, the first and second ultrasoundtransducers (37 and 39) may be arranged on the opposite walls of anexterior groove, instead of the internal mud channel, on the tool.

FIG. 7 is a prospective view showing an apparatus including first andsecond ultrasonic transducers (37 and 39) according to anotherembodiment of the invention. FIG. 8 shows the same apparatus in crosssection. The apparatus is shown as part of a tool (58) disposed in aborehole formed in a formation (57) such that an annulus exists betweenthe tool (58) and the borehole wall (55). The apparatus of thisembodiment uses a predetermined distance offset (ΔD_(f)) between thefront face (37 f) of the first transducer (37) and the front face (39 f)of the second transducer (39) for velocity calculation. An apparatus inthis configuration can be used to determine the velocity of anultrasound pulse in the annulus, even when the distance from the tool tothe borehole wall (55) is not known.

To determine the velocity of an ultrasound pulse using the apparatusshown in FIGS. 7 and 8, an ultrasound pulse is transmitted from each ofthe transducers (37 and 39), either simultaneously or in sequence. Thetime for each ultrasound pulse to travel a reflecting interface such asthe borehole wall (55) and back to the respective transducer thattransmitted the pulse is measured. The difference in the travel times(T₂−T₁) reflects the time it takes the ultrasound pulse, transmitted bythe transducer 37, farther from the reflecting interface, to traveltwice the predetermined offset distance (ΔD_(f)). The velocity of theultrasound pulse may be calculated by dividing 2 ΔD_(f) by thedifference in the travel times (T₂−T₁).

For the velocity measurement of this embodiment, several assumptionsshould be made: 1) the tool is parallel to the well axis; 2) the toolhas not moved with respect to the borehole wall in between the firings;3) the apparatus is reflecting approximately from the same isotropicacoustic-borehole-wall and there is no effect of rugosity; and 4) thediameter of the borehole does not change enough to cause amisinterpretation of the difference. Preferably, a spacing ofapproximately 5 cm or more is provided between the centers of thetransducers to minimize cross-talk. Although the formation (57) in FIGS.7 and 8 is shown as being made up of various layers for illustrativepurposes, for the purposes of the assumptions above it should beunderstood that the Figures are not to scale, and that the separationbetween the transducers is actually much smaller than the thickness of atypical formation layer. Thus, at any point in the borehole, it isassumed that both transducers are looking at the same layer of theformation.

Alternatively, a single ultrasound pulse may be emitted from either thefirst ultrasound transducer (37) or the second ultrasound transducer(39) and the reflected pulse (echo) is detected by both transducers (37)and (39). The difference between the times required for the reflectedpulse (echo) to travel back to the first ultrasound transducer (37) andthe second ultrasound transducer (39) corresponds to the time requiredfor the ultrasound pulse to travel a distance that equals thepredetermined offset (ΔD_(f)). In this case, the velocity of theultrasound pulse may be determined by dividing ΔD_(f) by the differencein the travel times (T₂-T₁).

The apparatus of this embodiment is useful for determining the velocityof an ultrasound pulse in the mud in the annulus. The mud in the annulusis frequently mixed with earth cuttings and/or formation fluids. Withthe ability to determine a precise velocity of an ultrasound pulse inthe mud in annulus, it becomes possible to infer the properties (e.g.,temperatures, pressure, compressibility, or formation fluidcontamination) of the mud in the annulus.

The apparatus shown in FIGS. 7 and 8 also may be used to determine aborehole diameter. Once the velocity of the ultrasound pulse isdetermined, the borehole diameter may be derived from the travel timesof the ultrasound pulses through the annulus. Because the diameter ofthe logging tool is known, the diameter of the borehole may bedetermined by adding to the latter the distances between the outer wallsof the tool and the inner wall of the borehole.

The borehole diameter may be determined in an alternative way by usingthe apparatus of this embodiment of the invention. Referring to thecross-sectional view of FIG. 8, the tool body (58) may be configured tohave two sections having different diameters (D₁ and D₂). The firstultrasound transducer (37) and the second ultrasound transducer (39) areeach located at a different section on the tool such that the front face(37 f) of the first ultrasound transducer (37) and the front face (39 f)of the second ultrasound transducer (39) are disposed at a predeterminedoffset ΔD_(f) that equals half the difference in the diameters of thetwo sections of the tool, ½(D₂−D₁). It is clear from FIG. 8 that:D _(bh) =D ₂+(V _(mud))(T ₁)/2  (1)andD _(bh) =D1+(D ₂ −D ₁)/2+(V _(mud))(T ₂)/2  (2)where D₁ is the diameter of the first section on the tool where theultrasound transducer (37) is located, D₂ is the diameter of the secondsection of the tool where the ultrasound transducer (39) is located,V_(mud) is the velocity of the ultrasound pulse, D_(bh) is the boreholediameter, and T₁ and T₂ are the two-way travel times measured by thefirst and second ultrasound transducers (37 and 39), respectively.Equations (1) and (2) may be rearranged to produce the followingrelationships:V _(mud)=(D ₂ −D)/(T ₂ −T ₁)  (3)andD _(bh) =D ₂+½T ₁[(D ₂ −D ₁)/(T ₂ −T ₁)]  (4)

Equation (3) can be used to derive the velocity of an ultrasound pulsefrom the difference in travel times (T₂−T₁) and the difference indiameters of the two sections of the tool (D₂−D₁). On the other hand,equation (4) may be used to derive the diameter of the borehole (53)without knowing the velocity of the ultrasound pulse. One skilled in theart would appreciate that it is also possible to use a phase difference(Δφ) between the two echoes, instead of the travel time difference(T₂−T₁), to calculate the velocity of the ultrasound pulse (V_(mud)) orthe distance to the target surface (d).

The methods and apparatus of the invention for determining the velocityof an ultrasound pulse as well as for measuring, for example, the radiusof a borehole, can be included in a great variety of downhole tools, forexample, a logging-while-drilling tool shown in FIG. 1.

For example, FIG. 9 shows a cross section of a pitch-catch ultrasounddevice incorporated as part of an LWD tool. Two ultrasound transducers(37 and 39) are included in the tool chassis (74) of an LWD tool and aredisposed across the mud channel (29). The ultrasound transducers (37 and39) are connected to downhole circuitry (not shown) for controlling theultrasound pulses and for recording the received signal as a function oftime.

FIG. 10 illustrates circuitry (82) for controlling the ultrasoundtransducers. As shown in FIG. 10, the circuitry (82) communicates withinternal tool communication bus (81) via an acquisition and businterface (83). The interface (83) connects a transmitter firing control(85), which obtains its power from a voltage converter and power supply(84). The transmitter firing control (85) controls the timing of theultrasound pulse emission from the ultrasound transmitter (86). Theultrasound pulse is detected by an ultrasound receiver (87). Thereceived signal is passed through a bandpass filter (88) and amplifiedby an amplifier (89). Finally, the signal is digitized by an analog todigital converter (ADC) (90) and the digitized signal is relayed by theinterface (83) to the internal tool communication bus (81). Thedigitized signal is stored in the memory in the tool for laterretrieval, processed by a downhole signal processor and/or immediatelycommunicated to a surface processor to compute the desired results(e.g., velocity of the ultrasound pulse, borehole diameter, etc).

The present invention has several advantages. For example, it eliminatesthe inaccuracy of estimating the velocity of an ultrasound pulse indownhole environment from a surface measurement. Embodiments of theinvention provide means for measuring the velocity of an ultrasoundpulse in the mud channel or in the annulus in the downhole environment.Accurate determination of the ultrasound velocity makes it possible toinfer mud properties (e.g., temperature, pressure, or compressibility)in the downhole environment.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having the benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein. Forexample, embodiments of the invention may be used with any acousticwave, not just ultrasound frequency. Accordingly, the scope of theinvention should be limited only by the attached claims.

1. A method for determining a velocity of ultrasound propagation in adrilling fluid flowing in a mud channel within a borehole of a downholeenvironment, comprising: disposing a first ultrasound transducer (37)across the mud channel from a second ultrasound transducer (39) suchthat a front face (37 f) of the first transducer (37) is offset from afront face (39 f) of the second ultrasound transducer (39) by apredetermined radial offset distance (ΔDf), wherein the transducers areseparated from the mud channel by a thin interface for protecting thetransducers from the drilling fluid flowing in the mud channel whilepermitting the ultrasound propagation there through; emitting anultrasound pulse into the drilling fluid in a borehole using the firstultrasound transducer(37); detecting the ultrasound pulse after theultrasound pulse has travelled through the drilling fluid a distance(d); determining a travel time (t) for the ultrasound pulse to travelthe distance (d) through the drilling fluid in the borehole between thefirst and second transducers; and determining the velocity of ultrasoundpropagation in the drilling fluid from the distance (d) and the traveltime (t).
 2. The method according to claim 1, wherein the detecting theultrasound pulse is performed with the first ultrasound transducer (37).3. The method according to claim 1, wherein the detecting the ultrasoundpulse is performed with the second ultrasound transducer (39).
 4. Themethod according to claim 1, wherein the detecting the ultrasound pulseis performed with both the first and second ultrasound transducer. 5.The method according to claim 4, further comprising determining aborehole diameter (D_(bh)) using the predetermined offset distance (ΔDf)and a difference in travel times (T₂−T₁) for the ultrasound pulse to bedetected by the first ultrasound transducer (37) and the secondultrasound transducer (39).
 6. The method according to claim 1, whereinthe detecting the ultrasound pulse is performed by the first ultrasoundtransducer (37), and wherein the method further comprises: emitting asecond ultrasound pulse into the drilling fluid in the borehole usingthe second ultrasound transducer (39); and detecting the secondultrasound pulse after the second ultrasound pulse has traveled throughthe drilling fluid a distance (d+2ΔD_(f)) using the second ultrasoundtransducer (39).
 7. The method according to claim 6, wherein theultrasound pulse and the second ultrasound pulse are emittedsimultaneously.
 8. The method according to claim 1, wherein the drillingfluid is located in an annulus between a tool and a borehole wall.
 9. Anapparatus for determining a velocity of ultrasound propagation in adrilling fluid within a borehole of a downhole environment, comprising:a first ultrasound transducer (37) disposed on a tool; a secondultrasound transducer (39) across the mud channel from a secondultrasound transducer (39) such that a front face (37 f) of the firsttransducer (37) is offset from a front face (39 f) of the secondultrasound transducer (39) by a predetermined radial offset distance(ΔDf), wherein the transducers are separated from the mud channel by athin interface for protecting the transducers from the drilling fluidflowing in the mud channel while permitting the ultrasound propagationthere through; and a circuitry (82) for controlling a timing of anultrasound pulse transmitted by the first ultrasound transducer (37) andfor measuring a time lapse between ultrasound transmission and detectionafter the ultrasound pulse has traveled a distance (d) through thedrilling fluid in the borehole between the first and second transducers.10. The apparatus according to claim 9, wherein the first ultrasoundtransducer (37) and the second ultrasound transducer (39) are disposedon an outside surface of the tool.
 11. Apparatus for determining avelocity of ultrasound propagation in drilling mud, the apparatuscomprising: a tool chassis located within a borehole, the chassis isshaped to define a mud channel therein for providing a path throughwhich the drilling mud is pumped into the borehole; a first and a secondultrasonic transducer located across the mud channel and facing eachother spaced at a distance (d), wherein the transducers are separatedfrom the mud channel by a thin interface for protecting the transducersfrom the drilling fluid flowing in the mud channel while permitting theultrasound propagation there through; circuitry for controlling thefirst and the second transducers to measure a time lapse betweenultrasound transmission and detection after an ultrasound pulse hastraveled the distance (d) across the drilling mud, and is thereby ableto determine the velocity of ultrasound propagation in the drilling mud.